1. Field of the Invention
The present invention relates to an exposure control circuit for a camera and more particularly to an improvement of such control circuit including an electromagnet to control a camera shutter or diaphragm.
2. Description of the Prior Art
In general, the attraction force of an electromagnet is not lost immediately after the current through its winding is shut off, but rather it decays exponentially with time. Further, when the electromagnet is turned off from its excited condition to demagnetization, the decay rate of its attraction force varies in accordance with the value of the current that had flowed through the winding during its excited condition.
The same phenomenon occurs also, but in opposed direction, when the electromagnet is turned on from its demagnetized condition to excitation. The increasing rate of the attraction varies then in accordance with the value of the current flowing into the winding.
Since winding current is proportional to source voltage with which the magnet is excited, we can consider this phenomenon as substituting the variation of source voltage for that of the winding current.
The above described phenomenon is of importance in particular for the control of a camera shutter or diaphragm with an electromagnet. To control the shutter or diaphragm, a power source cell having a very limited capacity is usually used and a stable control is always required. The phenomenon has adverse affects on the electromagnet controlling shutter or diaphragm.
As to the control of the camera shutter, there are known two types of control system. The one is of the type wherein the electromagnet is excited and its attraction force is used to prevent the shutter from closing. After a certain given time has passed, the electromagnet is demagnetized to release the shutter. In the other type of control system, the shutter is actuated to open while the electromagnet is in the demagnetized condition, and after a certain given time has passed, the electromagnet is excited to commence the closing action of the shutter.
In either type of electric shutter, the closing action of the shutter is actuated only when the attraction force of the electromagnet has reached a given level, whatever the shutter speed may be. Therefore, if the source voltage with which the electromagnet is excited, is reduced for any reason, for example, due to use-up or low temperature in a cold region, the variation of the source voltage will cause an error in shutter speed, in particular, on the high speed side. It is the same as to a control circuit in which camera diaphragm is controlled by an electromagnet.